With the increase over the past few years in computer processing speeds, there has been a desire for higher recording speeds and densities in magnetic recording and reproducing apparatus that record and retrieve data signals, such as hard disk drives (HDD). Current HDDs make use of a longitudinal magnetic recording method in which the direction of magnetization lies within the plane of the magnetic recording medium. However, to attain an even higher recording density in HDDs, a perpendicular magnetic recording method in which the direction of magnetization is oriented normal to the medium is advantageous because a sharp magnetization transition is achieved.
Moreover, with regard to the thermal fluctuation that has become a concern in recent magnetic recording media, because perpendicular magnetic recording technology allows the magnetic recording layer on the medium to be made thicker than would be possible with longitudinal magnetic recording technology, deterioration of the recorded signals can be minimized.
Research on perpendicular magnetic recording layers has been focused primarily on CoCr alloy-based (e.g., CoCrPti alloy) magnetic layers which have an irregular hexagonal closed packed (hcp) crystal structure. To address the problem of thermal fluctuation, considerable research has also been done on materials having a larger magnetic anisotropy (Ku).
However, because the magnetic anisotropy Ku is in direct proportion to the coercivity (Hc), a larger recording magnetic field is required to such materials having a large anisotropy Ku. Hence, in such perpendicular magnetic recording media, the anisotropy Ku is often set to a value close to the upper limit in the recording ability of the head.
Here, if the coercivity Hc could be lowered while keeping the anisotropy Ku large, it would be possible to obtain a perpendicular magnetic recording medium having a good resistance to thermal fluctuation.
Tilted perpendicular magnetic recording media (referred to below as “tilted media”) in which the direction of easy axis of the magnetization in the magnetic recording layer is tilted with respect to the direction normal to the medium have recently been described (e.g., see Patent Documents 1 and 2, and Non-Patent Document 1). That is, whereas in conventional perpendicular magnetic recording media the crystal planes of the magnetic crystal grains are oriented so that the easy axis directs normal to the medium, these tilted media are characterized in that the crystal planes of the magnetic crystal grains are oriented so that the easy axis is tilted with respect to the normal to the medium.
Non-Patent Document 1 reports the coercivity Hc decreases due to angle between the direction of the applied magnetic field, which is normal to the medium, and the direction of easy axis of the magnetization. Theoretically, it has been found that the smallest coercivity Hc can be achieved when the angle of the direction of the applied magnetic field and the direction of easy axis is 45°. The coercivity Hc achieved at an intersection angle of 45° is about one-half that when the angle is 0°.
To orient the direction of the easy axis of the magnetization in the magnetic recording layer so that it is tilted with respect to the normal to the medium, it is desirable to employ an under layer which enables the growth of magnetic crystal grains having such an orientation. However, little research has been done on such under layers for the CoCrPt alloy magnetic layers currently in practical use.
Furthermore, a practically useful tilted medium requires a “granular structure” in which the magnetic crystal grains are separated by a nonmagnetic material. Yet, in the CoCrPt alloy magnetic layers mentioned above, little research has been done on methods for achieving a granular structure in which the magnetic crystal grains are oriented so that the easy axis of the magnetization (C axis) is tilted with respect to the normal to the medium.
Hence, numerous problems need to be resolved for thin film formation in order to achieve tilted media using current alloy-based magnetic materials. Moreover, in such tilted media, the magnetic crystal grains are oriented at an angle, which undesirably lowers the output. If the C axis grows at a random angle, the problem of a de-magnetization field in the magnetic transition region will arise in the same way as in conventional longitudinal magnetic recording.
In tilted media, because the magnetization vectors for individual magnetic grains face in different directions, when producing patterned media wherein the recording data or recording track shapes are imparted to the magnetic recording layer, large variations arise between the magnetic characteristics in each pattern. In spite of this, an under layer that grows the magnetic grains to be oriented uniformly at an angle in the circumferential direction.
Another approach that has been described for achieving tilted media (e.g., see Non-Patent Document 2) involves a composite medium composed of, in the above-described granular structure, magnetically isolated hard magnetic grains which are exchange-coupled with similarly isolated soft magnetic grains. When this composite medium is in a state where a magnetic field has not been applied, magnetization of the magnetic grains overall is oriented in the perpendicular direction. When a recording magnetic field is applied, the soft magnetic layer undergo a magnetic reversal and makes the magnetization of the exchange-coupled hard magnetic layer tilt and then ultimately resulting in a tilted medium.
This composite medium eliminates the need to form the easy axis of the magnetization of the hard magnetic layer with a tilt, thus making it unnecessary to control the orientation of the magnetic crystal grains. However, because neither a method of forming a soft magnetic layer having a granular structure nor a method of achieving a good crystal orientation of the hard magnetic grain on the soft magnetic grain has yet to be established, there remain unsolved challenges in the fabrication of such a medium.
Also, in composite media, to obtain the above-described effect, it is necessary for the soft magnetic layer to have a large thickness. However, because magnetization by the individual magnetic grains becomes larger in such a case, magneto static coupling between the magnetic grains will affect the recordability or the stability of the recorded pattern. In addition, the material margin becomes narrower.    Patent Document 1: Japanese Unexamined Patent Application Publication JP-Hei8-129736A    Patent Document 2: Japanese Patent Publication JP-3235003B    Non-Patent Document 1: IEEE Transaction on Magnetics, Vol. 38, pp. 3675-3683.    Non-Patent Document 2: IEEE Transaction on Magnetics, Vol. 41, pp. 537.